On the role of Rossby wave phase speed for persistent temperature extremes

Case studies of mid-latitude summer heatwaves commonly regard stationary synoptic-scale Rossby waves as the primary dynamical forcing. Whether this relationship between upper-tropospheric Rossby wave phase speed and persistent temperature extremes can be generalized is less clear. Here, we evaluate interannual and intra-seasonal variability of Rossby wave phase speed in reanalysis datasets employing circumglobal spectral analysis and investigate episodes with a low or a high zonal phase speed, respectively. Locally, we find evidence of Rossby wave phase preferences during episodes with a low phase speed, where preferred locations of ridges coincide with regions of increased heatwave frequency, but globally, there is no indication of an increased heatwave frequency. Unexpectedly, the finding of Rossby wave phase preferences and increased heatwave frequency also hold for episodes with a high phase speed, although in different areas of the summer hemisphere mid-latitudes. These findings, in particular about episodes with a zonal phase speed, will improve our mechanistic understanding of the dynamical drivers of heatwaves.